A remote control apparatus of the above-described type suitable for locking and unlocking a vehicle door is called a keyless entry system or a smart entry system. For example, Japanese Patent Application Laid-Open No. 5-106376 discloses a system comprising a portable radio unit and a vehicle-mounted radio unit. The portable radio unit includes first transmission means which transmits a reply signal or response signal when first reception means receives a transmission request signal or call signal. The vehicle-mounted radio unit includes second transmission means for transmitting the call signal at predetermined intervals, and control means which outputs a signal for unlocking doors of a vehicle when second reception means receives the response signal and outputs a signal for locking doors of a vehicle after passage of a predetermined period of time during which the response signal is not received. When a driver carrying the portable radio unit approaches to the vehicle in order to enter the vehicle, the driver-side door is automatically unlocked, and when the driver exits and moves away from the vehicle, the door is automatically locked.
When the driver is outside the vehicle, the second transmission means transmits the call signal. That is, the call signal is transmitted when the engine of the vehicle is stopped. Accordingly, the transmission of the call signal imposes a load on a power source (battery) onboard the vehicle, so that battery electrical power is consumed. If the transmission interval of the call signal is increased in order to reduce power consumption and the communicable distance is relatively short, there arises a problem that the driver carrying the portable radio unit reaches the driver-side door from an incommunicable position during a period between transmission of a call signal and transmission of a next call signal, with the result that the door is not opened even though the driver attempts to open the door. This problem can be mitigated if the transmission power of the call signal is increased in order to increase the communicable distance. In this case, since the amount of consumed power per transmission of the call signal increases, this method is not suitable for reduction of battery power consumption. Accordingly, the transmission interval of the call signal cannot be simply increased.